Is it possible to add new airlines and routes in FSX? Let's say that I've download SK livery and want the AI to fly their routes with a specific aircraft. Eg CPH-NRT with A343 and ARN-LHR with B738. How do I do that?

You can always add routes by yourself, but that is a bit difficult to do and requires a certain amount of knowledge about ai traffic tools.
It is therefore more advisable to use complete AI packages as a beginning.
World of AI ( http://world-of-ai.com/index.php ) is highly popular among simmers due to their great variety of airlines (commerical, cargo, GA/Business, military) and the frame-saving repaints which look splendid at the same time. Basically, you have to download their installer and the packages you want to install.
If an airline has not yet been featured, you can visit their forum and request it, or find instructions how to create your own traffic using freeware from the well-known flightsim databases (like Avsim and flightsim).

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